The Prp19p-associated complex in spliceosome activation

Functional roles of protein splicing factors

RNA splicing is one of the fundamental processes in gene expression in eukaryotes. Splicing of pre-mRNA is catalysed by a large ribonucleoprotein complex called the spliceosome, which consists of five small nuclear RNAs and numerous protein factors. The spliceosome is a highly dynamic structure, assembled by sequential binding and release of the small nuclear RNAs and protein factors. DExD/H-box RNA helicases are required to mediate structural changes in the spliceosome at various steps in the assembly pathway and have also been implicated in the fidelity control of the splicing reaction. Other proteins also play key roles in mediating the progression of the spliceosome pathway. In this review, we discuss the functional roles of the protein factors involved in the spliceosome pathway primarily from studies in the yeast system.

The interaction of Prp2 with a defined region of the intron is required for the first splicing reaction

In Saccharomyces cerevisiae, the 3' splice site is not required for the first catalytic reaction of splicing. We have previously reported that at least 24 nucleotides downstream of the branch point is required for the first reaction to take place, but the precatalytic spliceosome forms efficiently on the truncated pre-mRNA with only 5 nucleotides retained downstream of the branch point. The factors that mediate this length-dependent control of the first catalytic step are not known. We show here that Prp2 can be recruited to the spliceosome without interacting with pre-mRNA when the 3' tail is short. Prp2 interacts with the intron when the 3' tail is extended, which results in destabilization of Prp2 and, consequently, progression of the first reaction. An RNA segment at 23 to 33 nucleotides downstream of the branch point is necessary and sufficient for the ATP-dependent action of Prp2. We also show that Prp2 directly interacts with the carboxyl-terminal fragment of Brr2 by pulldown assays. We propose that Prp2 is recruited to the spliceosome via interaction with Brr2 and is spatially positioned to interact with this specific region of the pre-mRNA, which stimulates the ATPase activity of Prp2 to promote the progression of the first catalytic step.

Cwc24p is a general Saccharomyces cerevisiae splicing factor required for the stable U2 snRNP binding to primary transcripts

Splicing of primary transcripts is an essential process for the control of gene expression. Specific conserved sequences in premature transcripts are important to recruit the spliceosome machinery. The Saccharomyces cerevisiae catalytic spliceosome is composed of about 60 proteins and 5 snRNAs (U1, U2, U4/U6 and U5). Among these proteins, there are core components and regulatory factors, which might stabilize or facilitate splicing of specific substrates. Assembly of a catalytic complex depends on the dynamics of interactions between these proteins and RNAs. Cwc24p is an essential S. cerevisiae protein, originally identified as a component of the NTC complex, and later shown to affect splicing in vivo. In this work, we show that Cwc24p also affects splicing in vitro. We show that Cwc24p is important for the U2 snRNP binding to primary transcripts, co-migrates with spliceosomes, and that it interacts with Brr2p. Additionally, we show that Cwc24p is important for the stable binding of Prp19p to the spliceosome. We propose a model in which Cwc24p is required for stabilizing the U2 association with primary transcripts, and therefore, especially important for splicing of RNAs containing non-consensus branchpoint sequences.

MRN1 implicates chromatin remodeling complexes and architectural factors in mRNA maturation

A functional relationship between chromatin structure and mRNA processing events has been suggested, however, so far only a few involved factors have been characterized. Here we show that rsc nhp6ΔΔ mutants, deficient for the function of the chromatin remodeling factor RSC and the chromatin architectural proteins Nhp6A/Nhp6B, accumulate intron-containing pre-mRNA at the restrictive temperature. In addition, we demonstrate that rsc8-ts16 nhp6ΔΔ cells contain low levels of U6 snRNA and U4/U6 di-snRNA that is further exacerbated after two hours growth at the restrictive temperature. This change in U6 snRNA and U4/U6 di-snRNA levels in rsc8-ts16 nhp6ΔΔ cells is indicative of splicing deficient conditions. We identify MRN1 (multi-copy suppressor of rsc nhp6ΔΔ) as a growth suppressor of rsc nhp6ΔΔ synthetic sickness. Mrn1 is an RNA binding protein that localizes both to the nucleus and cytoplasm. Genetic interactions are observed between 2 µm-MRN1 and the splicing deficient mutants snt309Δ, prp3, prp4, and prp22, and additional genetic analyses link MRN1, SNT309, NHP6A/B, SWI/SNF, and RSC supporting the notion of a role of chromatin structure in mRNA processing.

Ccdc94 protects cells from ionizing radiation by inhibiting the expression of p53

DNA double-strand breaks (DSBs) represent one of the most deleterious forms of DNA damage to a cell. In cancer therapy, induction of cell death by DNA DSBs by ionizing radiation (IR) and certain chemotherapies is thought to mediate the successful elimination of cancer cells. However, cancer cells often evolve to evade the cytotoxicity induced by DNA DSBs, thereby forming the basis for treatment resistance. As such, a better understanding of the DSB DNA damage response (DSB–DDR) pathway will facilitate the design of more effective strategies to overcome chemo- and radioresistance. To identify novel mechanisms that protect cells from the cytotoxic effects of DNA DSBs, we performed a forward genetic screen in zebrafish for recessive mutations that enhance the IR–induced apoptotic response. Here, we describe radiosensitizing mutation 7 (rs7), which causes a severe sensitivity of zebrafish embryonic neurons to IR–induced apoptosis and is required for the proper development of the central nervous system. The rs7 mutation disrupts the coding sequence of ccdc94, a highly conserved gene that has no previous links to the DSB–DDR pathway. We demonstrate that Ccdc94 is a functional member of the Prp19 complex and that genetic knockdown of core members of this complex causes increased sensitivity to IR–induced apoptosis. We further show that Ccdc94 and the Prp19 complex protect cells from IR–induced apoptosis by repressing the expression of p53 mRNA. In summary, we have identified a new gene regulating a dosage-sensitive response to DNA DSBs during embryonic development. Future studies in human cancer cells will determine whether pharmacological inactivation of CCDC94 reduces the threshold of the cancer cell apoptotic response.

Radiation therapy and most chemotherapies elicit cancer cell death through the induction of excessive DNA damage. However, cancer cells can harbor genetic defects that confer resistance to these therapies. To identify cellular components whose targeted therapeutic inactivation could potentially enhance the sensitivity of treatment-resistant cancer cells to DNA–damaging therapies, we have chosen an unbiased genetic approach in live whole zebrafish embryos to identify genes that normally protect cells from the lethal effects of DNA damage. This approach has yielded the discovery of a novel radioprotective gene called ccdc94. Upon inactivation of ccdc94, cells become more sensitive to radiation-induced cell death. Our further analysis revealed that the Ccdc94 protein functions in the Prp19 complex, which is known to regulate gene expression and repair of damaged DNA. We found that this complex normally represses radiation-induced cell death by inhibiting the expression of the p53 gene, a critical mediator of DNA damage–induced cell death. Future experiments that inactivate Ccdc94 and Prp19 complex proteins in human cancer cells will determine if inactivation of this complex represents a novel therapeutic strategy that could increase p53 expression to enhance sensitivity to DNA damaging therapies in chemo- and radio-resistant cancer cells.

CEF1/CDC5 alleles modulate transitions between catalytic conformations of the spliceosome

Conformational change within the spliceosome is required between the first and second catalytic steps of pre-mRNA splicing. A prior genetic screen for suppressors of an intron mutant that stalls between the two steps yielded both prp8 and non-prp8 alleles that suppressed second-step splicing defects. We have now identified the strongest non-prp8 suppressors as alleles of the NTC (Prp19 complex) component, CEF1. These cef1 alleles generally suppress second-step defects caused by a variety of intron mutations, mutations in U6 snRNA, or deletion of the second-step protein factor Prp17, and they can activate alternative 3' splice sites. Genetic and functional interactions between cef1 and prp8 alleles suggest that they modulate the same event(s) in the first-to-second-step transition, most likely by stabilization of the second-step spliceosome; in contrast, alleles of U6 snRNA that also alter this transition modulate a distinct event, most likely by stabilization of the first-step spliceosome. These results implicate a myb-like domain of Cef1/CDC5 in interactions that modulate conformational states of the spliceosome and suggest that alteration of these events affects splice site use, resulting in alternative splicing-like patterns in yeast.

Cwc2 and its human homologue RBM22 promote an active conformation of the spliceosome catalytic centre

RNA-structural elements play key roles in pre-mRNA splicing catalysis; yet, the formation of catalytically competent RNA structures requires the assistance of spliceosomal proteins. We show that the S. cerevisiae Cwc2 protein functions prior to step 1 of splicing, and it is not required for the Prp2-mediated spliceosome remodelling that generates the catalytically active B complex, suggesting that Cwc2 plays a more sophisticated role in the generation of a functional catalytic centre. In active spliceosomes, Cwc2 contacts catalytically important RNA elements, including the U6 internal stem-loop (ISL), and regions of U6 and the pre-mRNA intron near the 5' splice site, placing Cwc2 at/near the spliceosome's catalytic centre. These interactions are evolutionarily conserved, as shown by studies with Cwc2's human counterpart RBM22, indicating that Cwc2/RBM22-RNA contacts are functionally important. We propose that Cwc2 induces an active conformation of the spliceosome's catalytic RNA elements. Thus, the function of RNA-RNA tertiary interactions within group II introns, namely to induce an active conformation of domain V, may be fulfilled by proteins that contact the functionally analogous U6-ISL, within the spliceosome.

Crystal structure of Cwc2 reveals a novel architecture of a multipartite RNA-binding protein

The yeast splicing factor Cwc2 contacts several catalytically important RNA elements in the active spliceosome, suggesting that Cwc2 is involved in determining their spatial arrangement at the spliceosome's catalytic centre. We have determined the crystal structure of the Cwc2 functional core, revealing how a previously uncharacterized Torus domain, an RNA recognition motif (RRM) and a zinc finger (ZnF) are tightly integrated in a compact folding unit. The ZnF plays a pivotal role in the architecture of the whole assembly. UV-induced crosslinking of Cwc2-U6 snRNA allowed the identification by mass spectrometry of six RNA-contacting sites: four in or close to the RRM domain, one in the ZnF and one on a protruding element connecting the Torus and RRM domains. The three distinct regions contacting RNA are connected by a contiguous and conserved positively charged surface, suggesting an expanded interface for RNA accommodation. Cwc2 mutations confirmed that the connector element plays a crucial role in splicing. We conclude that Cwc2 acts as a multipartite RNA-binding platform to bring RNA elements of the spliceosome's catalytic centre into an active conformation.

Structure of the mRNA splicing complex component Cwc2: insights into RNA recognition

The Prp19-associated complex [NTC (nineteen complex)] plays a crucial role in intron removal during premature mRNA splicing in eukaryotes. Only one component of the NTC, Cwc2, is capable of binding RNA. In the present study we report the 1.9 Å (1 Å=0.1 nm) X-ray structure of the Cwc2 core domain, which is both necessary and sufficient for RNA binding. The Cwc2 core domain contains two sub-domains, a CCCH-type ZnF (zinc finger) and a RRM (RNA recognition motif). Unexpectedly, the ZnF domain and the RRM form a single folding unit, glued together by extensive hydrophobic interactions and hydrogen bonds. Structure-guided mutational analysis revealed that the intervening loop [known as the RB loop (RNA-binding loop)] between ZnF and RRM plays an essential role in RNA binding. In addition, a number of highly conserved positively charged residues on the β-strands of RRM make an important contribution to RNA binding. Intriguingly, these residues and a portion of the RB loop constitute an extended basic surface strip that encircles Cwc2 halfway. The present study serves as a framework for understanding the regulatory function of the NTC in RNA splicing.

A quantitative high-throughput in vitro splicing assay identifies inhibitors of spliceosome catalysis

Despite intensive research, there are very few reagents with which to modulate and dissect the mRNA splicing pathway. Here, we describe a novel approach to identify such tools, based on detection of the exon junction complex (EJC), a unique molecular signature that splicing leaves on mRNAs. We developed a high-throughput, splicing-dependent EJC immunoprecipitation (EJIPT) assay to quantitate mRNAs spliced from biotin-tagged pre-mRNAs in cell extracts, using antibodies to EJC components Y14 and eukaryotic translation initiation factor 4aIII (eIF4AIII). Deploying EJIPT we performed high-throughput screening (HTS) in conjunction with secondary assays to identify splicing inhibitors. We describe the identification of 1,4-naphthoquinones and 1,4-heterocyclic quinones with known anticancer activity as potent and selective splicing inhibitors. Interestingly, and unlike previously described small molecules, most of which target early steps, our inhibitors represented by the benzothiazole-4,7-dione, BN82685, block the second of two trans-esterification reactions in splicing, preventing the release of intron lariat and ligation of exons. We show that BN82685 inhibits activated spliceosomes' elaborate structural rearrangements that are required for second-step catalysis, allowing definition of spliceosomes stalled in midcatalysis. EJIPT provides a platform for characterization and discovery of splicing and EJC modulators.

Saccharomyces cerevisiae NineTeen complex (NTC)-associated factor Bud31/Ycr063w assembles on precatalytic spliceosomes and improves first and second step pre-mRNA splicing efficiency

Pre-mRNA splicing occurs in spliceosomes whose assembly and activation are critical for splice site selection and catalysis. The highly conserved NineTeen complex protein complex stabilizes various snRNA and protein interactions early in the spliceosome assembly pathway. Among several NineTeen complex-associated proteins is the nonessential protein Bud31/Ycr063w, which is also a component of the Cef1p subcomplex. A role for Bud31 in pre-mRNA splicing is implicated by virtue of its association with splicing factors, but its specific functions and spliceosome interactions are uncharacterized. Here, using in vitro splicing assays with extracts from a strain lacking Bud31, we illustrate its role in efficient progression to the first catalytic step and its requirement for the second catalytic step in reactions at higher temperatures. Immunoprecipitation of functional epitope-tagged Bud31 from in vitro reactions showed that its earliest association is with precatalytic B complex and that the interaction continues in catalytically active complexes with stably bound U2, U5, and U6 small nuclear ribonucleoproteins. In complementary experiments, wherein precatalytic spliceosomes are selected from splicing reactions, we detect the occurrence of Bud31. Cross-linking of proteins to pre-mRNAs with a site-specific 4-thio uridine residue at the -3 position of exon 1 was tested in reactions with WT and bud31 null extracts. The data suggest an altered interaction between a ∼25-kDa protein and this exonic residue of pre-mRNAs in the arrested bud31 null spliceosomes. These results demonstrate the early spliceosomal association of Bud31 and provide plausible functions for this factor in stabilizing protein interactions with the pre-mRNA.

The spliceosome-activating complex: molecular mechanisms underlying the function of a pleiotropic regulator

Correct interpretation of the coding capacity of RNA polymerase II transcribed eukaryotic genes is determined by the recognition and removal of intronic sequences of pre-mRNAs by the spliceosome. Our current knowledge on dynamic assembly and subunit interactions of the spliceosome mostly derived from the characterization of yeast, Drosophila, and human spliceosomal complexes formed on model pre-mRNA templates in cell extracts. In addition to sequential structural rearrangements catalyzed by ATP-dependent DExH/D-box RNA helicases, catalytic activation of the spliceosome is critically dependent on its association with the NineTeen Complex (NTC) named after its core E3 ubiquitin ligase subunit PRP19. NTC, isolated recently from Arabidopsis, occurs in a complex with the essential RNA helicase and GTPase subunits of the U5 small nuclear RNA particle that are required for both transesterification reactions of splicing. A compilation of mass spectrometry data available on the composition of NTC and spliceosome complexes purified from different organisms indicates that about half of their conserved homologs are encoded by duplicated genes in Arabidopsis. Thus, while mutations of single genes encoding essential spliceosome and NTC components lead to cell death in other organisms, differential regulation of some of their functionally redundant Arabidopsis homologs permits the isolation of partial loss of function mutations. Non-lethal pleiotropic defects of these mutations provide a unique means for studying the roles of NTC in co-transcriptional assembly of the spliceosome and its crosstalk with DNA repair and cell death signaling pathways.

Structure of the LSm657 complex: an assembly intermediate of the LSm1-7 and LSm2-8 rings

The nuclear LSm2-8 (like Sm) complex and the cytoplasmic LSm1-7 complex play a central role in mRNA splicing and degradation, respectively. The LSm proteins are related to the spliceosomal Sm proteins that form a heteroheptameric ring around small nuclear RNA. The assembly process of the heptameric Sm complex is well established and involves several smaller Sm assembly intermediates. The assembly of the LSm complex, however, is less well studied. Here, we solved the 2.5 Å-resolution structure of the LSm assembly intermediate that contains LSm5, LSm6, and LSm7. The three monomers display the canonical Sm fold and arrange into a hexameric LSm657-657 ring. We show that the order of the LSm proteins within the ring is consistent with the order of the related SmE, SmF, and SmG proteins in the heptameric Sm ring. Nonetheless, differences in RNA binding pockets prevent the prediction of the nucleotide binding preferences of the LSm complexes. Using high-resolution NMR spectroscopy, we confirm that LSm5, LSm6, and LSm7 also assemble into a 60-kDa hexameric ring in solution. With a combination of pull-down and NMR experiments, we show that the LSm657 complex can incorporate LSm23 in order to assemble further towards native LSm rings. Interestingly, we find that the NMR spectra of the LSm57, LSm657-657, and LSm23-657 complexes differ significantly, suggesting that the angles between the LSm building blocks change depending on the ring size of the complex. In summary, our results identify LSm657 as a plastic and functional building block on the assembly route towards the LSm1-7 and LSm2-8 complexes.

Trans-splicing in trypanosomes: machinery and its impact on the parasite transcriptome

In trypanosomes, all RNAs are processed by the concerted action of trans-splicing and polyadenylation. In trans-splicing, a common spliced leader (SL) is donated to all mRNAs from a small RNA molecule, the SL RNA. This article summarizes recent findings in the field focusing on SL RNA transcription, cap modifications and pseudouridylation. The role(s) of these modifications for splicing and gene expression are discussed. The recruitment of SL RNA to the spliceosome depends on splicing factors and recent progress in identifying such factors is described. A recent major advance in understanding the role of trans-splicing in the trypanosome transcriptome was obtained by whole-genome mapping of the SL and polyadenylation sites, revealing surprising heterogeneity and suggesting that gene regulation, especially during cycling between the two hosts of the parasite, involves alternative trans-splicing. Finally, the SL silencing mechanism, which is harnessed by the parasite to control gene expression under stress, is discussed.

Spliceosome structure and function

Pre-mRNA splicing is catalyzed by the spliceosome, a multimegadalton ribonucleoprotein (RNP) complex comprised of five snRNPs and numerous proteins. Intricate RNA-RNA and RNP networks, which serve to align the reactive groups of the pre-mRNA for catalysis, are formed and repeatedly rearranged during spliceosome assembly and catalysis. Both the conformation and composition of the spliceosome are highly dynamic, affording the splicing machinery its accuracy and flexibility, and these remarkable dynamics are largely conserved between yeast and metazoans. Because of its dynamic and complex nature, obtaining structural information about the spliceosome represents a major challenge. Electron microscopy has revealed the general morphology of several spliceosomal complexes and their snRNP subunits, and also the spatial arrangement of some of their components. X-ray and NMR studies have provided high resolution structure information about spliceosomal proteins alone or complexed with one or more binding partners. The extensive interplay of RNA and proteins in aligning the pre-mRNA's reactive groups, and the presence of both RNA and protein at the core of the splicing machinery, suggest that the spliceosome is an RNP enzyme. However, elucidation of the precise nature of the spliceosome's active site, awaits the generation of a high-resolution structure of its RNP core.

The RNA polymerase II C-terminal domain promotes splicing activation through recruitment of a U2AF65-Prp19 complex

Pre-mRNA splicing is frequently coupled to transcription by RNA polymerase II (RNAPII). This coupling requires the C-terminal domain of the RNAPII largest subunit (CTD), although the underlying mechanism is poorly understood. Using a biochemical complementation assay, we previously identified an activity that stimulates CTD-dependent splicing in vitro. We purified this activity and found that it consists of a complex of two well-known splicing factors: U2AF65 and the Prp19 complex (PRP19C). We provide evidence that both U2AF65 and PRP19C are required for CTD-dependent splicing activation, that U2AF65 and PRP19C interact both in vitro and in vivo, and that this interaction is required for activation of splicing. Providing the link to the CTD, we show that U2AF65 binds directly to the phosphorylated CTD, and that this interaction results in increased recruitment of U2AF65 and PRP19C to the pre-mRNA. Our results not only provide a mechanism by which the CTD enhances splicing, but also describe unexpected interactions important for splicing and its coupling to transcription.

The Prp19 complex is a novel transcription elongation factor required for TREX occupancy at transcribed genes

Different steps in gene expression are intimately linked. In Saccharomyces cerevisiae, the conserved TREX complex couples transcription to nuclear messenger RNA (mRNA) export. However, it is unknown how TREX is recruited to actively transcribed genes. Here, we show that the Prp19 splicing complex functions in transcription elongation. The Prp19 complex is recruited to transcribed genes, interacts with RNA polymerase II (RNAPII) and TREX, and is absolutely required for TREX occupancy at transcribed genes. Importantly, the Prp19 complex is necessary for full transcriptional activity. Taken together, we identify the Prp19 splicing complex as a novel transcription elongation factor that is essential for TREX occupancy at transcribed genes and that thus provides a novel link between transcription and messenger ribonucleoprotein (mRNP) formation.

Semiquantitative proteomic analysis of the human spliceosome via a novel two-dimensional gel electrophoresis method

More than 200 proteins associate with human spliceosomes, but little is known about their relative abundances in a given spliceosomal complex. Here we describe a novel two-dimensional (2D) electrophoresis method that allows separation of high-molecular-mass proteins without in-gel precipitation and thus without loss of protein. Using this system coupled with mass spectrometry, we identified 171 proteins altogether on 2D maps of stage-specific spliceosomal complexes. By staining with a fluorescent dye with a wide linear intensity range, we could quantitate and categorize proteins as present in high, moderate, or low abundance. Affinity-purified human B, B(act), and C complexes contained 69, 63, and 72 highly/moderately abundant proteins, respectively. The recruitment and release of spliceosomal proteins were followed based on their abundances in A, B, B(act), and C spliceosomal complexes. Staining with a phospho-specific dye revealed that approximately one-third of the proteins detected in human spliceosomal complexes by 2D gel analyses are phosphorylated. The 2D gel electrophoresis system described here allows for the first time an objective view of the relative abundances of proteins present in a particular spliceosomal complex and also sheds additional light on the spliceosome's compositional dynamics and the phosphorylation status of spliceosomal proteins at specific stages of splicing.

Breaking up the C complex spliceosome shows stable association of proteins with the lariat intron intermediate

Spliceosome assembly requires several structural rearrangements to position the components of the catalytic core. Many of these rearrangements involve successive strengthening and weakening of different RNA:RNA and RNA:proteins interactions within the complex. To gain insight into the organization of the catalytic core of the spliceosome arrested between the two steps of splicing chemistry (C complex), we investigated the effects of exposing C complex to low concentrations of urea. We find that in the presence of 3M urea C complex separates into at least three sub-complexes. One sub-complex contains the 5'exon, another contains the intron-lariat intermediate, and U2/U5/U6 snRNAs likely comprise a third sub-complex. We purified the intron-lariat intermediate sub-complex and identified several proteins, including U2 snRNP and PRP19 complex (NTC) components. The data from our study indicate that U2 snRNP proteins in C complex are more stably associated with the lariat-intron intermediate than the U2 snRNA. The results also suggest a set of candidate proteins that hold the lariat-intron intermediate together in C complex. This information is critical for further interpreting the complex architecture of the mammalian spliceosome.

Systematic two-hybrid and comparative proteomic analyses reveal novel yeast pre-mRNA splicing factors connected to Prp19

Prp19 is the founding member of the NineTeen Complex, or NTC, which is a spliceosomal subcomplex essential for spliceosome activation. To define Prp19 connectivity and dynamic protein interactions within the spliceosome, we systematically queried the Saccharomyces cerevisiae proteome for Prp19 WD40 domain interaction partners by two-hybrid analysis. We report that in addition to S. cerevisiae Cwc2, the splicing factor Prp17 binds directly to the Prp19 WD40 domain in a 1:1 ratio. Prp17 binds simultaneously with Cwc2 indicating that it is part of the core NTC complex. We also find that the previously uncharacterized protein Urn1 (Dre4 in Schizosaccharomyces pombe) directly interacts with Prp19, and that Dre4 is conditionally required for pre-mRNA splicing in S. pombe. S. pombe Dre4 and S. cerevisiae Urn1 co-purify U2, U5, and U6 snRNAs and multiple splicing factors, and dre4Δ and urn1Δ strains display numerous negative genetic interactions with known splicing mutants. The S. pombe Prp19-containing Dre4 complex co-purifies three previously uncharacterized proteins that participate in pre-mRNA splicing, likely before spliceosome activation. Our multi-faceted approach has revealed new low abundance splicing factors connected to NTC function, provides evidence for distinct Prp19 containing complexes, and underscores the role of the Prp19 WD40 domain as a splicing scaffold.

Dynamic histone acetylation is critical for cotranscriptional spliceosome assembly and spliceosomal rearrangements

Assembly of the spliceosome onto pre-mRNA is a dynamic process involving the ordered exchange of snRNPs to form the catalytically active spliceosome. These ordered rearrangements have recently been shown to occur cotranscriptionally, while the RNA polymerase is still actively engaged with the chromatin template. We previously demonstrated that the histone acetyltransferase Gcn5 is required for U2 snRNP association with the branchpoint. Here we provide evidence that histone acetylation and deacetylation facilitate proper cotranscriptional association of spliceosomal snRNPs. As with GCN5, mutation or deletion of Gcn5-targeted histone H3 residues leads to synthetic lethality when combined with deletion of the genes encoding the U2 snRNP components Lea1 or Msl1. Gcn5 associates throughout intron-containing genes and, in the absence of the histone deacetylases Hos3 and Hos2, enhanced histone H3 acetylation is observed throughout the body of genes. Deletion of histone deacetylaces also results in persistent association of the U2 snRNP and a severe defect in the association of downstream factors. These studies show that cotranscriptional spliceosome rearrangements are driven by dynamic changes in the acetylation state of histones and provide a model whereby yeast spliceosome assembly is tightly coupled to histone modification.

DEAH-box ATPase Prp16 has dual roles in remodeling of the spliceosome in catalytic steps

The assembly of the spliceosome involves dynamic rearrangements of interactions between snRNAs, protein components, and the pre-mRNA substrate. DExD/H-box ATPases are required to mediate structural changes of the spliceosome, utilizing the energy of ATP hydrolysis. Two DExD/H-box ATPases are required for the catalytic steps of the splicing pathway, Prp2 for the first step and Prp16 for the second step, both belonging to the DEAH subgroup of the protein family. The detailed mechanism of their action was not well understood until recently, when Prp2 was shown to be required for the release of U2 components SF3a and SF3b, presumably to allow the binding of Cwc25 to promote the first transesterification reaction. We show here that Cwc25 and Yju2 are released after the reaction in Prp16- and ATP-dependent manners, possibly to allow for the binding of Prp22, Prp18, and Slu7 to promote the second catalytic reaction. The binding of Cwc25 to the spliceosome is destabilized by mutations at the branchpoint sequence, suggesting that Cwc25 may bind to the branch site. We also show that Prp16 has an ATP-independent role in the first catalytic step, in addition to its known role in the second step. In the absence of ATP, Prp16 stabilizes the binding of Cwc25 to the spliceosome formed with branchpoint mutated pre-mRNAs to facilitate their splicing. Our results uncovered novel functions of Prp16 in both catalytic steps, and provide mechanistic insights into splicing catalysis.

Two putative RNA-binding proteins function with unequal genetic redundancy in the MOS4-associated complex

The MOS4-associated complex (MAC) is a highly conserved nuclear protein complex associated with the spliceosome. We recently purified the MAC from Arabidopsis (Arabidopsis thaliana) nuclei, identified its potential components by mass spectrometry, and showed that at least five core proteins in the MAC are required for defense responses in plants. Here, we report the characterization of a putative RNA-binding protein identified in the MAC named MAC5A and its close homolog MAC5B. We confirmed that MAC5A is a component of the MAC through coimmunoprecipitation with the previously described MAC protein CELL DIVISION CYCLE5 from Arabidopsis. In addition, like all other characterized MAC proteins, MAC5A fused to the Green Fluorescent Protein localizes to the nucleus. Double mutant analysis revealed that MAC5A and MAC5B are unequally redundant and that a double mac5a mac5b mutant results in lethality. Probably due to this partial redundancy, mac5a and mac5b single mutants do not exhibit enhanced susceptibility to virulent or avirulent pathogen infection. However, like other MAC mutations, mac5a-1 partially suppresses the autoimmune phenotypes of suppressor of npr1-1, constitutive1 (snc1), a gain-of-function mutant that expresses a deregulated Resistance protein. Our results suggest that MAC5A is a component of the MAC that contributes to snc1- mediated autoimmunity.

Characterization of purified human Bact spliceosomal complexes reveals compositional and morphological changes during spliceosome activation and first step catalysis

To better understand the compositional and structural dynamics of the human spliceosome during its activation, we set out to isolate spliceosomal complexes formed after precatalytic B but prior to catalytically active C complexes. By shortening the polypyrimidine tract of the PM5 pre-mRNA, which lacks a 3' splice site and 3' exon, we stalled spliceosome assembly at the activation stage. We subsequently affinity purified human B(act) complexes under the same conditions previously used to isolate B and C complexes, and analyzed their protein composition by mass spectrometry. A comparison of the protein composition of these complexes allowed a fine dissection of compositional changes during the B to B(act) and B(act) to C transitions, and comparisons with the Saccharomyces cerevisiae B(act) complex revealed that the compositional dynamics of the spliceosome during activation are largely conserved between lower and higher eukaryotes. Human SF3b155 and CDC5L were shown to be phosphorylated specifically during the B to B(act) and B(act) to C transition, respectively, suggesting these modifications function at these stages of splicing. The two-dimensional structure of the human B(act) complex was determined by electron microscopy, and a comparison with the B complex revealed that the morphology of the human spliceosome changes significantly during its activation. The overall architecture of the human and S. cerevisiae B(act) complex is similar, suggesting that many of the higher order interactions among spliceosomal components, as well as their dynamics, are also largely conserved.

The spliceosomal proteome: at the heart of the largest cellular ribonucleoprotein machine

Almost all primary transcripts in higher eukaryotes undergo several splicing events and alternative splicing is a major factor in generating proteomic diversity. Thus, the spliceosome, the ribonucleoprotein assembly that performs splicing, is a highly critical cellular machine and as expected, a very complex one. Indeed, the spliceosome is one of the largest, if not the largest, molecular machine in the cell with over 150 different components in human. A large fraction of the spliceosomal proteome is organized into small nuclear ribonucleoprotein particles by associating with one of the small nuclear RNAs, and the function of many spliceosomal proteins revolve around their association or interaction with the spliceosomal RNAs or the substrate pre-messenger RNAs. In addition to the complex web of protein-RNA interactions, an equally complex network of protein-protein interactions exists in the spliceosome, which includes a number of large, conserved proteins with critical functions in the spliceosomal catalytic core. These include the largest conserved nuclear protein, Prp8, which plays a critical role in spliceosomal function in a hitherto unknown manner. Taken together, the large spliceosomal proteome and its dynamic nature has made it a highly challenging system to study, and at the same time, provides an exciting example of the evolution of a proteome around a backbone of primordial RNAs likely dating from the RNA World.

Splicing factor Cwc22 is required for the function of Prp2 and for the spliceosome to escape from a futile pathway

Cwc22 was previously identified to associate with the pre-mRNA splicing factor Cef1/Ntc85, a component of the Prp19-associated complex (nineteen complex [NTC]) involved in spliceosome activation. We show here that Cwc22 is required for pre-mRNA splicing both in vivo and in vitro but is neither tightly associated with the NTC nor required for spliceosome activation. Cwc22 is associated with the spliceosome prior to catalytic steps and remains associated throughout the reaction. The stable association of Cwc22 with the spliceosome requires the presence of the NTC but is independent of Prp2. Although Cwc22 is not required for the recruitment of Prp2 to the spliceosome, it is essential for the function of Prp2 in promoting the release of the U2 components SF3a and SF3b. In the absence of Cwc22, Prp2 can bind to the spliceosome but is dissociated upon ATP hydrolysis without promoting the release of SF3a/b. Thus, Cwc22 represents a novel ATP-dependent step one factor besides Prp2 and Spp2 and has a distinct role from that of Spp2 in mediating the function of Prp2.

Protein arginine methylation facilitates cotranscriptional recruitment of pre-mRNA splicing factors

Cotranscriptional recruitment of pre-mRNA splicing factors to their genomic targets facilitates efficient and ordered assembly of a mature messenger ribonucleoprotein particle (mRNP). However, how the cotranscriptional recruitment of splicing factors is regulated remains largely unknown. Here, we demonstrate that protein arginine methylation plays a novel role in regulating this process in Saccharomyces cerevisiae. Our data show that Hmt1, the major type I arginine methyltransferase, methylates Snp1, a U1 small nuclear RNP (snRNP)-specific protein, and that the mammalian Snp1 homolog, U1-70K, is likewise arginine methylated. Genome-wide localization analysis reveals that the deletion of the HMT1 gene deregulates the recruitment of U1 snRNP and its associated components to intron-containing genes (ICGs). In the same context, splicing factors acting downstream of U1 snRNP addition bind to a reduced number of ICGs. Quantitative measurement of the abundance of spliced target transcripts shows that these changes in recruitment result in an increase in the splicing efficiency of developmentally regulated mRNAs. We also show that in the absence of either Hmt1 or of its catalytic activity, an association between Snp1 and the SR-like protein Npl3 is substantially increased. Together, these data support a model whereby arginine methylation modulates dynamic associations between SR-like protein and pre-mRNA splicing factor to promote target specificity in splicing.

The function of spliceosome components in open mitosis

Spatial separation of eukaryotic cells into the nuclear and cytoplasmic compartment permits uncoupling of DNA transcription from translation of mRNAs and allows cells to modify newly transcribed pre mRNAs extensively. Intronic sequences (introns), which interrupt the coding elements (exons), are excised ("spliced") from pre-mRNAs in the nucleus to yield mature mRNAs. This not only enables alternative splicing as an important source of proteome diversity, but splicing is also an essential process in all eukaryotes and knock-out or knock-down of splicing factors frequently results in defective cell proliferation and cell division. However, higher eukaryotes progress through cell division only after breakdown of the nucleus ("open mitosis"). Open mitosis suppresses basic nuclear functions such as transcription and splicing, but allows separate, mitotic functions of nuclear proteins in cell division. Mitotic defects arising after loss-of-function of splicing proteins therefore could be an indirect consequence of compromised splicing in the closed nucleus of the preceding interphase or reflect a direct contribution of splicing proteins to open mitosis. Although experiments to directly distinguish between these two alternatives have not been reported, indirect evidence exists for either hypotheses. In this review, we survey published data supporting an indirect function of splicing in open mitosis or arguing for a direct function of spliceosomal proteins in cell division.

The function of the NineTeen Complex (NTC) in regulating spliceosome conformations and fidelity during pre-mRNA splicing

The NineTeen Complex (NTC) of proteins associates with the spliceosome during pre-mRNA splicing and is essential for both steps of intron removal. The NTC and other NTC-associated proteins are recruited to the spliceosome where they participate in regulating the formation and progression of essential spliceosome conformations required for the two steps of splicing. It is now clear that the NTC is an integral component of active spliceosomes from yeast to humans and provides essential support for the spliceosomal snRNPs (small nuclear ribonucleoproteins). In the present article, we discuss the identification and characterization of the yeast NTC and review recent work in yeast that supports the essential role for this complex in the regulation and fidelity of splicing.

The Prp19 complex and the Usp4Sart3 deubiquitinating enzyme control reversible ubiquitination at the spliceosome

The spliceosome, a dynamic assembly of proteins and RNAs, catalyzes the excision of intron sequences from nascent mRNAs. Recent work has suggested that the activity and composition of the spliceosome are regulated by ubiquitination, but the underlying mechanisms have not been elucidated. Here, we report that the spliceosomal Prp19 complex modifies Prp3, a component of the U4 snRNP, with nonproteolytic K63-linked ubiquitin chains. The K63-linked chains increase the affinity of Prp3 for the U5 snRNP component Prp8, thereby allowing for the stabilization of the U4/U6.U5 snRNP. Prp3 is deubiquitinated by Usp4 and its substrate targeting factor, the U4/U6 recycling protein Sart3, which likely facilitates ejection of U4 proteins from the spliceosome during maturation of its active site. Loss of Usp4 in cells interferes with the accumulation of correctly spliced mRNAs, including those for alpha-tubulin and Bub1, and impairs cell cycle progression. We propose that the reversible ubiquitination of spliceosomal proteins, such as Prp3, guides rearrangements in the composition of the spliceosome at distinct steps of the splicing reaction.

Repair of DNA interstrand cross-links during S phase of the mammalian cell cycle

DNA interstrand cross-linking (ICL) agents are widely used in anticancer chemotherapy regimens, yet our understanding of the DNA repair mechanisms by which these lesions are removed from the genome remains incomplete. This is at least in part due to the enormously complicated nature and variety of the biochemical pathways that operate on these complex lesions. In this review, we have focused specifically on the S-phase pathway of ICL repair in mammalian cells, which appears to be the major mechanism by which these lesions are removed in cycling cells. The various stages and components of this pathway are discussed, and a putative molecular model is presented. In addition, we propose an explanation as to how this pathway can lead to the observed high levels of sister chromatid exchanges known to be induced by ICLs.

Analysis of spliceosomal proteins in Trypanosomatids reveals novel functions in mRNA processing

In trypanosomatids, all mRNAs are processed via trans-splicing, although cis-splicing also occurs. In trans-splicing, a common small exon, the spliced leader (SL), which is derived from a small SL RNA species, is added to all mRNAs. Sm and Lsm proteins are core proteins that bind to U snRNAs and are essential for both these splicing processes. In this study, SmD3- and Lsm3-associated complexes were purified to homogeneity from Leishmania tarentolae. The purified complexes were analyzed by mass spectrometry, and 54 and 39 proteins were purified from SmD3 and Lsm complexes, respectively. Interestingly, among the proteins purified from Lsm3, no mRNA degradation factors were detected, as in Lsm complexes from other eukaryotes. The U1A complex was purified and mass spectrometry analysis identified, in addition to U1 small nuclear ribonucleoprotein (snRNP) proteins, additional co-purified proteins, including the polyadenylation factor CPSF73. Defects observed in cells silenced for U1 snRNP proteins suggest that the U1 snRNP functions exclusively in cis-splicing, although U1A also participates in polyadenylation and affects trans-splicing. The study characterized several trypanosome-specific nuclear factors involved in snRNP biogenesis, whose function was elucidated in Trypanosoma brucei. Conserved factors, such as PRP19, which functions at the heart of every cis-spliceosome, also affect SL RNA modification; GEMIN2, a protein associated with SMN (survival of motor neurons) and implicated in selective association of U snRNA with core Sm proteins in trypanosomes, is a master regulator of snRNP assembly. This study demonstrates the existence of trypanosomatid-specific splicing factors but also that conserved snRNP proteins possess trypanosome-specific functions.

The Prp19 WD40 domain contains a conserved protein interaction region essential for its function

Prp19 is a member of the WD40 repeat family of E3 ubiquitin ligases and a conserved eukaryotic RNA splicing factor essential for activation and stabilization of the spliceosome. To understand the role of the WD40 repeat domain of Prp19 we have determined its structure using X-ray crystallography. The domain has a distorted seven bladed WD40 architecture with significant asymmetry due to irregular packing of blades one and seven into the core of the WD40 domain. Structure-based mutagenesis identified a highly conserved surface centered around blade five that is required for the physical interaction between Prp19 and Cwc2, another essential splicing factor. This region is found to be required for Prp19 function and yeast viability. Experiments in vitro and in vivo demonstrate that two molecules of Cwc2 bind to the Prp19 tetramer. These coupled structural and functional studies provide a model for the functional architecture of Prp19.

Exon definition complexes contain the tri-snRNP and can be directly converted into B-like precatalytic splicing complexes

The first step in splicing of pre-mRNAs with long introns is exon definition, where U1 and U2 snRNPs bind at opposite ends of an exon. After exon definition, these snRNPs must form a complex across the upstream intron to allow splicing catalysis. Exon definition and conversion of cross-exon to cross-intron spliceosomal complexes are poorly understood. Here we demonstrate that, in addition to U1 and U2 snRNPs, cross-exon complexes contain U4, U5, and U6 (which form the tri-snRNP). Tri-snRNP docking involves the formation of U2/U6 helix II. This interaction is stabilized by a 5' splice site (SS)-containing oligonucleotide, which can bind the tri-snRNP and convert the cross-exon complex into a cross-intron, B-like complex. Our data suggest that the switch from cross-exon to cross-intron complexes can occur directly when an exon-bound tri-snRNP interacts with an upstream 5'SS, without prior formation of a cross-intron A complex, revealing an alternative spliceosome assembly pathway.

Release of SF3 from the intron branchpoint activates the first step of pre-mRNA splicing

Eukaryotic pre-mRNA splicing is a complex process requiring the precise timing and action of >100 trans-acting factors. It has been known for some time that the two steps of splicing chemistry require three DEAH-box RNA helicase-like proteins; however, their mechanism of action at these steps has remained elusive. Spliceosomes arrested in vivo at the three helicase checkpoints were purified, and first step-arrested spliceosomes were functionally characterized. We show that the first step of splicing requires a novel ATP-independent conformational change. Prp2p then catalyzes an ATP-dependent rearrangement displacing the SF3a and SF3b complexes from the branchpoint within the spliceosome. We propose a model in which SF3 prevents premature nucleophilic attack of the chemically reactive hydroxyl of the branchpoint adenosine prior to the first transesterification. When the spliceosome attains the proper conformation and upon the function of Prp2p, SF3 is displaced from the branchpoint allowing first step chemistry to occur.

Molecular architecture of the human Prp19/CDC5L complex

Protein complexes containing Prp19 play a central role during catalytic activation of the spliceosome, and Prp19 and its related proteins are major components of the spliceosome's catalytic core RNP. To learn more about the spatial organization of the human Prp19 (hPrp19)/CDC5L complex, which is comprised of hPrp19, CDC5L, PRL1, AD002, SPF27, CTNNBL1, and HSP73, we purified native hPrp19/CDC5L complexes from HeLa cells stably expressing FLAG-tagged AD002 or SPF27. Stoichiometric analyses indicated that, like Saccharomyces cerevisiae NTC (nineteen complex), the human Prp19/CDC5L complex contains four copies of hPrp19. Salt treatment identified a stable core comprised of CDC5L, hPrp19, PRL1, and SPF27. Protein-protein interaction studies revealed that SPF27 directly interacts with each component of the hPrp19/CDC5L complex core and also elucidated several additional, previously unknown interactions between hPrp19/CDC5L complex components. Limited proteolysis of the hPrp19/CDC5L complex revealed a protease-resistant complex comprised of SPF27, the C terminus of CDC5L, and the N termini of PRL1 and hPrp19. Under the electron microscope, purified hPrp19/CDC5L complexes exhibit an elongated, asymmetric shape with a maximum dimension of approximately 20 nm. Our findings not only elucidate the molecular organization of the hPrp19/CDC5L complex but also provide insights into potential protein-protein interactions at the core of the catalytically active spliceosome.

The evolutionarily conserved core design of the catalytic activation step of the yeast spliceosome

Metazoan spliceosomes exhibit an elaborate protein composition required for canonical and alternative splicing. Thus, the minimal set of proteins essential for activation and catalysis remains elusive. We therefore purified in vitro assembled, precatalytic spliceosomal complex B, activated B(act), and step 1 complex C from the simple eukaryote Saccharomyces cerevisiae. Mass spectrometry revealed that yeast spliceosomes contain fewer proteins than metazoans and that each functional stage is very homogeneous. Dramatic compositional changes convert B to B(act), which is composed of approximately 40 evolutionarily conserved proteins that organize the catalytic core. Additional remodeling occurs concomitant with step 1, during which nine proteins are recruited to form complex C. The moderate number of proteins recruited to complex C will allow investigations of the chemical reactions in a fully defined system. Electron microscopy reveals high-quality images of yeast spliceosomes at defined functional stages, indicating that they are well-suited for three-dimensional structure analyses.

A large intrinsically disordered region in SKIP and its disorder-order transition induced by PPIL1 binding revealed by NMR

Intrinsically disordered proteins or protein regions play an important role in fundamental biological processes. During spliceosome activation, a large structural rearrangement occurs. The Prp19 complex and related factors are involved in the catalytic activation of the spliceosome. Recent mass spectrometric analyses have shown that Ski interaction protein (SKIP) and peptidylprolyl isomerase-like protein 1 (PPIL1) are Prp19-related factors that constitute the spliceosome B, B*, and C complexes. Here, we report that a highly flexible region of SKIP (SKIPN, residues 59-129) is intrinsically disordered. Upon binding to PPIL1, SKIPN undergoes a disorder-order transition. A highly conserved fragment of SKIP (residues 59-79) called the PPIL1-binding fragment (PBF) was sufficient to bind PPIL1. The structure of PBF.PPIL1 complex, solved by NMR, shows that PBF exhibits an ordered structure and interacts with PPIL1 through electrostatic and hydrophobic interactions. Three subfragments in the PBF (residues 59-67, 68-73, and 74-79) show hook-like backbone structure, and interactions between these subfragments are necessary for PBF.PPIL1 complex formation. PPIL1 is a cyclophilin family protein. It is recruited by SKIP into the spliceosome by a region other than the peptidylprolyl isomerase active site. This enables the active site of PPIL1 to remain open in the complex and still function as a peptidylprolyl cis/trans-isomerase or molecular chaperon to facilitate the folding of other proteins in the spliceosomes. The large disordered region in SKIP provides an interaction platform. Its disorder-order transition, induced by PPIL1 binding, may adapt the requirement for a large structural rearrangement occurred in the activation of spliceosome.

Physical and genetic interactions of yeast Cwc21p, an ortholog of human SRm300/SRRM2, suggest a role at the catalytic center of the spliceosome

In Saccharomyces cerevisiae, Cwc21p is a protein of unknown function that is associated with the NineTeen Complex (NTC), a group of proteins involved in activating the spliceosome to promote the pre-mRNA splicing reaction. Here, we show that Cwc21p binds directly to two key splicing factors-namely, Prp8p and Snu114p-and becomes the first NTC-related protein known to dock directly to U5 snRNP proteins. Using a combination of proteomic techniques we show that the N-terminus of Prp8p contains an intramolecular fold that is a Snu114p and Cwc21p interacting domain (SCwid). Cwc21p also binds directly to the C-terminus of Snu114p. Complementary chemical cross-linking experiments reveal reciprocal protein footprints between the interacting Prp8 and Cwc21 proteins, identifying the conserved cwf21 domain in Cwc21p as a Prp8p binding site. Genetic and functional interactions between Cwc21p and Isy1p indicate that they have related functions at or prior to the first catalytic step of splicing, and suggest that Cwc21p functions at the catalytic center of the spliceosome, possibly in response to environmental or metabolic changes. We demonstrate that SRm300, the only SR-related protein known to be at the core of human catalytic spliceosomes, is a functional ortholog of Cwc21p, also interacting directly with Prp8p and Snu114p. Thus, the function of Cwc21p is likely conserved from yeast to humans.

A systematic characterization of Cwc21, the yeast ortholog of the human spliceosomal protein SRm300

Cwc21 (complexed with Cef1 protein 21) is a 135 amino acid yeast protein that shares homology with the N-terminal domain of human SRm300/SRRM2, a large serine/arginine-repeat protein shown previously to associate with the splicing coactivator and 3'-end processing stimulatory factor, SRm160. Proteomic analysis of spliceosomal complexes has suggested a role for Cwc21 and SRm300 at the core of the spliceosome. However, specific functions for these proteins have remained elusive. In this report, we employ quantitative genetic interaction mapping, mass spectrometry of tandem affinity-purified complexes, and microarray profiling to investigate genetic, physical, and functional interactions involving Cwc21. Combined data from these assays support multiple roles for Cwc21 in the formation and function of splicing complexes. Consistent with a role for Cwc21 at the core of the spliceosome, we observe strong genetic, physical, and functional interactions with Isy1, a protein previously implicated in the first catalytic step of splicing and splicing fidelity. Together, the results suggest multiple functions for Cwc21/SRm300 in the splicing process, including an important role in the activation of splicing in association with Isy1.

Reconstitution of both steps of Saccharomyces cerevisiae splicing with purified spliceosomal components

The spliceosome is a ribonucleoprotein machine that removes introns from pre-mRNA in a two-step reaction. To investigate the catalytic steps of splicing, we established an in vitro splicing complementation system. Spliceosomes stalled before step 1 of this process were purified to near-homogeneity from a temperature-sensitive mutant of the RNA helicase Prp2, compositionally defined, and shown to catalyze efficient step 1 when supplemented with recombinant Prp2, Spp2 and Cwc25, thereby demonstrating that Cwc25 has a previously unknown role in promoting step 1. Step 2 catalysis additionally required Prp16, Slu7, Prp18 and Prp22. Our data further suggest that Prp2 facilitates catalytic activation by remodeling the spliceosome, including destabilizing the SF3a and SF3b proteins, likely exposing the branch site before step 1. Remodeling by Prp2 was confirmed by negative stain EM and image processing. This system allows future mechanistic analyses of spliceosome activation and catalysis.

Cwc25 is a novel splicing factor required after Prp2 and Yju2 to facilitate the first catalytic reaction

Cwc25 has previously been identified to associate with pre-mRNA splicing factor Cef1/Ntc85, a component of the Prp19-associated complex (nineteen complex, or NTC) involved in spliceosome activation. We show here that Cwc25 is neither tightly associated with NTC nor required for spliceosome activation but is required for the first catalytic reaction. The affinity-purified spliceosome formed in Cwc25-depleted extracts contained only pre-mRNA and could be chased into splicing intermediates upon the addition of recombinant Cwc25 in an ATP-independent manner, suggesting that Cwc25 functions in the final step of the first catalytic reaction after the action of Prp2. Yju2 and a heat-resistant factor of unknown identity, HP, have previously been shown to be required for the same step of the splicing pathway. Cwc25, although resistant to heat treatment, is not sufficient to replace the function of HP, indicating that another heat-resistant factor, which we named HP-X, is involved. The requirement of Cwc25 and HP-X for the first catalytic reaction could be partially compensated for when the affinity-purified spliceosome was incubated in the presence of low concentrations of Mn(2+). These results have implications for the possible roles of Cwc25 and HP-X in facilitating juxtaposition of the 5' splice site and the branch point during the first catalytic reaction.

Ntc90 is required for recruiting first step factor Yju2 but not for spliceosome activation

The Prp19-associated complex (NineTeen Complex [NTC]) is required for spliceosome activation by specifying interactions of U5 and U6 with pre-mRNA on the spliceosome after the release of U4. The NTC consists of at least eight protein components, including two tetratricopeptide repeat (TPR)-containing proteins, Ntc90 and Ntc77. Ntc90 has nine copies of the TPR with seven clustered in the carboxy-terminal half of the protein, and interacts with all identified NTC components except for Prp19 and Ntc25. It forms a stable complex with Ntc31, Ntc30, and Ntc20 in the absence of Ntc25, when other interactions between NTC components are disrupted. In this study, we used both biochemical and genetic methods to analyze the structure of Ntc90, and its function in maintaining the integrity of the NTC and in NTC-mediated spliceosome activation. Our results show that Ntc90 interacts with Ntc31, Ntc30, and other NTC components through different regions of the protein, and that its function may be regulated by Ntc31 and Ntc30. Ntc90 is not required for the association of Prp19, Ntc85, Ntc77, Ntc25, and Ntc20, or for their binding to the spliceosome. It is also not required for NTC-mediated spliceosome activation, but is required for the recruitment of Yju2, which is involved in the first catalytic reaction after the function of Prp2. Our results demonstrate a novel role of the NTC in recruiting splicing factors to the spliceosome after its activation.

Two Prp19-like U-box proteins in the MOS4-associated complex play redundant roles in plant innate immunity

Plant Resistance (R) proteins play an integral role in defense against pathogen infection. A unique gain-of-function mutation in the R gene SNC1, snc1, results in constitutive activation of plant immune pathways and enhanced resistance against pathogen infection. We previously found that mutations in MOS4 suppress the autoimmune phenotypes of snc1, and that MOS4 is part of a nuclear complex called the MOS4-Associated Complex (MAC) along with the transcription factor AtCDC5 and the WD-40 protein PRL1. Here we report the immuno-affinity purification of the MAC using HA-tagged MOS4 followed by protein sequence analysis by mass spectrometry. A total of 24 MAC proteins were identified, 19 of which have predicted roles in RNA processing based on their homology to proteins in the Prp19-Complex, an evolutionarily conserved spliceosome-associated complex containing homologs of MOS4, AtCDC5, and PRL1. Among these were two highly similar U-box proteins with homology to the yeast and human E3 ubiquitin ligase Prp19, which we named MAC3A and MAC3B. MAC3B was recently shown to exhibit E3 ligase activity in vitro. Through reverse genetics analysis we show that MAC3A and MAC3B are functionally redundant and are required for basal and R protein-mediated resistance in Arabidopsis. Like mos4-1 and Atcdc5-1, mac3a mac3b suppresses snc1-mediated autoimmunity. MAC3 localizes to the nucleus and interacts with AtCDC5 in planta. Our results suggest that MAC3A and MAC3B are members of the MAC that function redundantly in the regulation of plant innate immunity.

Proline-rich sequence recognition: I. Marking GYF and WW domain assembly sites in early spliceosomal complexes

Proline-rich sequences (PRS) and their recognition domains have emerged as transposable protein interaction modules during eukaryotic evolution. They are especially abundant in proteins associated with pre-mRNA splicing and likely assist in the formation of the spliceosome by binding to GYF and WW domains. Here we profile PRS-mediated interactions of the CD2BP2/52K GYF domain by a site-specific peptide inhibitor and stable isotope labeling/mass spectrometry analysis. Several PRS hubs with multiple proline-rich motifs exist that can recruit GYF and/or WW domains. Saturating the PRS sites by an isolated GYF domain inhibited splicing at the level of A complex formation. The interactions mediated by PRS are therefore important to the early phases of spliceosomal assembly.